A variety of methods have been developed for measurement of mechanical motion. For example, synchros and resolvers are low impedance devices that are used to generate analog signals representative of motor position and are capable of driving a synchro-to-digital converter which converts the analog signal to a digital signal representative of position for use in a servo system. These devices require an alternating current (AC) reference excitation signal. The time-phase difference between the excitation signals in the rotor and stator of these devices may be used to develop a position signal proportional to the angular position of the rotor with respect to the stator. This position signal is representative of the spatial phase angle of the motor and is ideally independent of the frequency and amplitude of the excitation signal. Undesirable effects that decrease the accuracy and precision of synchro/resolver position measurement systems are harmonic distortion, time-phase shifts due to capacitance, cable length, etc., and nonlinearities due to mechanical imperfections in the magnetics and windings of the synchro/resolver. Because of these practical limitations to the accuracy and precision of synchro-resolver devices, other types of position encoders are used where greater accuracy and precision are required.
Rotary position encoders achieve higher accuracy than synchro/resolver systems and are relatively economical. Typically, rotary encoders are comprised of chrome on glass and are highly invariant with temperature changes. Even higher accuracy can be achieved by converting rotary motion to linear motion using precision-machined mechanical devices. However, such linear encoders are costly and unsuitable for many applications. Optical position encoders can also provide high accuracy. However, these encoder require tight alignment tolerances and clean environments, making them expensive and difficult to implement. Magnetic linear encoders are less sensitive to alignment tolerances but are as expensive as optical encoders and require additional mounting bracketry.
What is needed is a relatively inexpensive method for measurement of mechanical position with high precision and accuracy that is easy to implement.